The full horror of the devastating earthquake and tsunami in Japan is starting to emerge amid fears that the death toll could run into many thousands. A day of high tension saw workers battle to save a nuclear plant from meltdown and 50,000 rescuers fight their way to victims in the midst of mud, flood waters, collapsed buildings and continuing blazes.

At least 1,700 people were reported dead or missing following the earthquake and tsunami, Kyodo news agency said. Further shocks of up to 6.4 magnitude continued to strike the north-east a day and a half after the major quake.

In addition, Kyodo said, 9,500 people could not be contacted in Minami Sanriku in the northern prefecture of Miyagi, around half the population. Japanese broadcaster NHK said 2,700 homes had been destroyed in Arahama, in the same prefecture. Further north, the National Police Agency said 5,000 homes were under water in Rikuzen-Takata, Iwate prefecture. Survivors were reported to be clambering over uprooted trees and overturned cars to reach homes. Rail operators were also searching for four commuter trains that were travelling coastal lines in Miyagi and Iwate when the tsunami struck.

The frantic search for survivors was almost overshadowed by the spectre of radioactive leaks at a nuclear power plant at the heart of the area most affected by Friday's earthquake. At the Fukushima plant, radiation leaked from a damaged reactor after an explosion blew the roof off. Japan's nuclear safety agency said the accident was rated less serious than the Three Mile Island or Chernobyl disasters, but up to 160 people were exposed to radiation.

Authorities told the International Atomic Energy Agency (IAEA), the UN's atomic watchdog, they were making preparations to distribute iodine to people living nearby. Iodine can increase resistance against thyroid cancer in the case of radioactive exposure.

A 20km evacuation zone was imposed around the plant as authorities moved tens of thousands of residents from the area, some 240km north of Tokyo, as they tried to reduce pressure in the reactors. The IAEA said Japanese authorities had informed it of the explosion and that they were "assessing the condition of the reactor core".

Early this morning, technicians were battling to relieve pressure in a second reactor at the plant after its cooling system failed. The procedure was expected to release a small amount of radiation.

As the first wave of military rescuers began arriving, prime minister Naoto Kan said 50,000 troops had joined rescue and recovery efforts, aided by boats and helicopters. Dozens of countries also offered help; Britain sent a specialist team of search and rescue experts. Andrew Mitchell, the international development secretary, said: "Our thoughts are with the people of Japan as they begin to recover from this terrible disaster."

Barack Obama pledged US assistance following what he called a potentially "catastrophic" disaster.

In a day of chaos, 300,000 people were evacuated from their homes in the region due to radiation and tsunami fears and quake damage. Two hundred fires had been sparked, including three that still raged in the north-east city of Kesennuma, Kyodo said. In the city of Shiogama, oil was leaking from a refinery into the harbour.

Friday's 8.9 magnitude shock, the fifth most powerful quake in a century, appeared to have moved the main island of Japan by three metres and shifted the Earth on its axis, an expert with the US Geological Survey told CNN.

As temperatures dropped to freezing in the worst-hit north-east prefectures, survivors huddled in shelters and braced themselves against more tremors. While many remained calm, some clutched friends and sobbed as the extent of the disaster sank in. TV footage showed rescue helicopters winching children and adults to safety from the wreckage. Many could be seen trapped on rooftops.

The city of Sendai remained swamped by water. Fishing boats, cars and buildings were stranded across farmland along the north-east coastline. "Everything is so hard," said Kumi Onodera, a 34-year-old local, who described her ordeal as "like a scene from a disaster movie. The road was moving up and down like a wave. Things were on fire and it was snowing [ash]."

Long queues formed outside shops in towns near the coastline as people cleared shelves of water and food. At least a million people in the north-east of the country were thought to be without drinking water. Five million homes are without power and electricity providers have warned they will need to impose rolling blackouts in parts of Tokyo.

There was worldwide consternation at the prospect of a nuclear meltdown in the wake of the Japanese earthquake. Tens of thousands of people took part in an anti-nuclear demonstration in southern Germany. The event had been planned for some time, but after the news of Japan's nuclear emergency, organisers were overwhelmed by some 50,000 people who turned up to take part.

The demonstrators, who stretched in a 45km chain from Neckarwestheim power plant to the city of Stuttgart, were demanding that the German government move away from nuclear power.

In the UK, energy secretary Chris Huhne said that the government was monitoring the nuclear situation in Japan. "It's too early to say what the cause was, let alone what the implications are," he added.

Many in the government and the private energy sector in the UK worry that the raising of the spectre of nuclear disaster will have implications for coalition plans to build 10 new nuclear power stations to replace Britain's ageing reactors.